20 



APPLIED ENTOMOLOGY 



food which having been digested has passed through the canal walls. Likewise 

 in passing over any organ needing this food, it is given up to those organs. The 

 blood therefore serves as a distributor of food from the place where it is digested 

 to all the parts which need it. 



We have already seen that the Hving parts of the body — the cells — need 

 oxygen, and as the result of their activities give off carbon dioxid gas, but that 

 this exchange is accomplished by the aid of the tracheae. In a somewhat parallel 

 way, the cells which need food obtain it from the blood. The cells by their 



Fig. 26. — Diagram showing by the direction of tlie arrows the general course of the blood 

 flow in a Dragon fly nymph, a, aorta; h, heart. {Modified from Kolbe.) 



activities produce not only carbonic acid gas but also waste material nitrogenous 

 in nature which must be removed like all wastes, from the body. This nitroge- 

 nous waste is picked up at the cells by the blood and carried along , perhaps for 

 some time before a place to dispose of it can be found. Sooner or later, however, 

 a particle of blood containing this waste material will wash over certain structures 

 called Malpighian tubes, to be described in the next section, and the cells which 

 form these tubes have the power to collect this waste material from the blood as 

 it flows over them, thus purifying it. 



The blood itself is usually a colorless (though sometimes yellowish, reddish or 

 greenish) fluid, in which are corpuscles resembling the white corpuscles of human 

 blood. It appears to serve to carry food to the tissues, and waste matter from 

 them, and therefore has no need of structures in it like the red blood corpuscles 

 of man, the work of which in insects is done by the trachea?. 



